As show FIG. 1, in the LED back light module at present, the LED temperature is very high, and the light guide plate will be expanded and deformed because of the high temperature. Thus, the light coupling distance between the light incident surface of the light guide plate and the light emergent surface of the LED light source is changed, and the LED incident light efficiency is seriously affected. In addition, the disappearance of the light coupling distance even causes the LED(s) to be pressed so that the LED lamp(s) fails or is damaged.